1. Field of the Invention
The present invention relates to a flameproof feed-through, and more particularly, to a flameproof feed-through providing control of impedance.
2. Statement of the Problem
Vibrating conduit sensors, such as Coriolis mass flowmeters and vibrating densitometers, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness and damping characteristics of the containing conduit and the material contained therein.
A typical Coriolis mass flowmeter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries, emulsions, and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes, including for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Two such transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the actuator. The two pickoff sensors are connected to electronic instrumentation. The instrumentation receives signals from the two pickoff sensors and processes the signals in order to derive a mass flow rate measurement, among other things. Vibratory flowmeters, including Coriolis mass flowmeters and densitometers, therefore employ one or more flow tubes that are vibrated in order to measure a fluid.
In some environments, electrical signals may need to be conducted through a flameproof physical barrier. For example, a flameproof physical barrier may separate the compartments of a fieldmount transmitter housing. Process control transmitters designed for use in hazardous atmospheres often utilize a combination of protection methods, including flameproof housings and/or barriers, to avoid uncontrolled explosions of flammable gases. International standards define the compliance requirements for flameproof devices and structures.
In the case of Coriolis flowmeter transmitters, it is well known to enclose the active electronics components within a flameproof compartment, so that an explosion of gases that might occur as a result of electrical energy within the electronics will not propagate beyond the enclosure. Furthermore, it is sometimes preferred that user-accessible connection facilities of the electronics utilize “increased safety” rather than flameproof as a protection method, wherein the connection facilities are shown to be non-sparking and therefore incapable of igniting a flammable gas. Under either standard, active electronics which could cause ignition are contained in a compartment wherein any ignition within the compartment cannot escape the compartment.
In order to provide electrical connectivity between the two compartments, a flameproof feed-through is employed. A common prior art flameproof feed-through is a cemented joint bushing. In a cemented joint bushing, a cemented joint may be formed between the conductors and the bushing casing or a cemented joint may be formed between a conductor insulation layer and the bushing casing. In a non-cemented joint, a small-tolerance interface may be used between the bushing casing and compartment wall, including joint interfaces to threaded, spigot, and other bushing casings. In order to be approved as flameproof, both types of joints must meet specific requirements, such as a temperature index rating and chemical compatibility, exceedingly tight tolerances (such as on the order of 0.1 or 0.15 millimeter, for example), and thread count, depth, and tolerance on a threaded joint.
FIG. 2 shows a prior art spigot-type feed-through. Wires, pins, or other conductors are cast into and pass through a feed-through body. A circumferential surface of the spigot-type feed-through body substantially contacts the inner surface of the aperture to form what is termed a spigot joint. A gap, and therefore a flame path, exists between the outer surface of the body and the inner surface of the aperture. The spigot-type feed-through body must achieve a minimal gap so that an unacceptably large gap and unacceptably short flame path are not allowed to exist.
The spigot-type feed-through body may comprise a potting material, such as a plastic. The potting material may be formed in the shape of the aperture and allowed to cure or harden before being assembled into the aperture as a spigot joint feed-through.
FIG. 3 shows a prior art flange-type feed-through. A flange-type feed-through body is positioned over and blocks an aperture. The flange-type feed-through body is larger than the aperture and overlaps the aperture. A gap and flame path comprises a radially directed path, extending outward from the axis between the flange-type feed-through body and an external surface of the barrier, housing, or other structure including the aperture.
All these approaches share certain disadvantages. First, the presence of discrete conductors presents a limitation on the ability to control the characteristic impedance of the electrical connections. As a result, the signal frequency that the connections can effectively carry is limited by the impedance of the prior art feed-through. Second, the process of manufacturing a feed-through involves the application and curing of cement or the need for a plastic over-molding process in order to form the physical barrier around the discrete conductors. These steps increase manufacturing time, complexity, and cost when creating an acceptable prior art flameproof feed-through.